In wireless communication systems, effective transmission/reception schemes and utilization schemes have been proposed in order to maximize the efficiency of limited radio resources. One of systems that are being taken into consideration in the next-generation wireless communication systems is a multi-carrier system. A multi-carrier system means a system for configuring a wide band by aggregating one or more carriers, each having a smaller bandwidth than a target wide band, when a wireless communication system tries to support a wide band.
A conventional wireless communication system, such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), uses carriers having various bandwidths, but the system was a single carrier system. In contrast, the next-generation wireless communication system, such as LTE-Advanced (LTE-A), can be a multi-carrier system using an aggregation of a plurality of carriers.
In a multi-carrier system, user equipment can receive a plurality of data units through a plurality of downlink carriers and feed a plurality of pieces of reception acknowledgement information, that is, acknowledgement/non-acknowledgement (ACK/NACK), for the plurality of data units back to a base station.
A multi-carrier system can operate either 1) in Frequency Division Duplex (FDD) in which uplink transmission and downlink transmission can be performed in different frequency bands at the same time or 2) in Time Division Duplex (TDD) in which uplink transmission and downlink transmission can be performed on different times, that is, in different subframes, in the same frequency band. If a multi-carrier system operates in TDD, the multi-carrier system may have to send ACK/NACKs for data units received through a plurality of downlink subframes for each of a plurality of downlink component carriers (DL CC) in one uplink subframe of an uplink component carrier (UL CC). In this case, the amount of ACK/NACK information fed back by user equipment is more increased than that of a conventional single carrier system.
Accordingly, there is a need for a method and apparatus for transmitting ACK/NACK different from an existing method of transmitting ACK/NACK that is used when a single carrier system operates in TDD.